1. Field of the Invention
The present invention relates to microwave systems, typically communications systems, such as find use in the communications band of 21.8 to 23.2 GHz carrier frequency. The invention further relates to a novel mixer for effecting a stable, low cost, single conversion of a microwave signal to an intermediate frequency suitably low (50 MHz) for convenient amplification. In a mixer operating at these disparate frequencies, efficient transmission paths and efficient filtering must be provided. The disclosed transmission paths include waveguides, microstrip circuits, transitions between waveguide and microstrip circuits, and suitable filters connected before and after the mixer.
2. Description of the Prior Art
A need has arisen for broadband, short range, low cost and generally directional point to point communications channels. Typically, such channels require adequate bandwidths for television channel capability or for wideband data transmission suited for computer applications. Higher microwave frequencies, i.e., "K" band, are now acessible with the advent of relatively low cost Gunn diode oscillators. (These currently operate up to about 30 GHz.) The Gunn diode oscillator provides both a low cost source of modulatable RF energy for low power signal transmission, and a low cost local oscillator for use in the signal conversion process. The Gunn diode oscillator is of a highly stable design, and permits use of relatively low IF frequencies, usually without the need for a closed loop frequency control. Thus, a simple, single conversion system is practical. Low cost dish antennas and efficient antenna waveguide feeds are well known (see Sichak article infra). In addition, low cost, high frequency diodes are available and packaged suitably for use as balanced mixers at these frequencies. In the mixer design, however, the question arises as to the most efficient and most economical method of providing the essential filtering and of providing the essential signal transmission paths. It is at this point that the use of microstrip circuits is suggested. Known microstrip circuits provide low cost construction of transmission paths, hybrid couplers, and filter circuits. These developments find application to the present invention.